A well known differential amplifier comprises a matched pair of bipolar transistors, a matched pair of load impedances and a current source. Each load impedance is connected between a voltage supply and a collector of a respective one of the transistors, and emitters of both transistors are connected to the current source. A differential input is applied to bases of the transistors, and a differential output is taken from the collectors of the transistors.
Two such differential amplifiers can be cross-coupled to construct a quadrature phase Voltage Controlled Oscillator (VCO) which oscillates with 90 degree phase shift across each differential amplifier and with 180 degree phase shift in the cross-coupled feedback loop. Unfortunately, the quality factors (Q) of such VCOs are smaller than desired for many high frequency applications.